Joint replacement surgery (arthroplasty) is often performed on the knee. In a total knee arthroplasty (TKA), the diseased cartilage surfaces of the thighbone (femur), the shinbone (tibia) and the kneecap (patella) are replaced by prosthetic components. Most of the other structures of the knee, such as the connecting ligaments, remain intact. This surgical procedure requires alignment of the femoral and tibial components to a vertical or mechanical axis of the limb. The procedure also requires enlarging the canal in the femur called a femoral medulla as well as enlarging the canal in the tibia called the tibial medulla. When the femoral and tibia bones are fully extended (i.e., the knee joint is in extension), a proximal-distal axis drawn through the center of a femoral head (proximal femur) passes through the knee joint in a healthy knee and along the tibial canal to the ankle joint. This proximal-distal axis is called the mechanical axis, and it is along this axis that a load is transmitted. However, the axis of the femoral medulla may lie at an angle of up to 7 degrees to this mechanical axis along the coronal plane. The femoral medulla has an anterior bow along a plane parallel to the sagittal plane. The bow represents a concavity facing in the posterior direction and serves to increase the space for lodgment of the soft tissue masses during knee flexion. Knee flexion refers to rotation of the femur with respect to the tibia. As part of a surgical procedure to replace the distal femur, a surgical device called an intramedullary (IM) rod is inserted into the femoral medulla. A cutting block is then mounted onto the rod and placed against the distal portion of the femur. The cutting block provides cutting guide surfaces for making the required cuts on the distal femur such as distal, posterior, anterior, posterior chamfer and anterior chamfer cuts. It is important that the rod provide an accurate reference for the cutting block.